Among the analogs of riboflavin that we have synthesized, two are utilized by rats for their flavoprotein enzymes when either is the only flavin available. The activities of the two homologs for several enzymes are very like that of riboflavin; however, in the case of succinic acid dehydrogenase (SDH), the activities of the two homologs and the vitamin are remarkably different. One of the homologs causes a substantial retardation (50%) in the growth rate of Walker rat carcinoma 256. This reduction correlates with the loss of the activity of SDH in these tumors. These differences can be best explained by the possible differences in the abilities of the three flavins to become covalently bonded to the enzyme. The purpose of this project is to synthesize these two homologs labeled with 14C and administer them to experimental animals. The radioactivity of the flavin covalently bonded to the enzyme in the mitochondria of the tumor can be determined following the removal of all flavin not covalently bonded. The radioactivity would be correlated with the enzyme activity. To compare with our accumulated data on the influence of these two flavins on the enzymes of the liver, the same evaluation would be made of the bonded flavin in the mitochondria of the liver. Besides SDH, the mammalian mitochondrion also contains monamine oxidase (MAO). This MAO also bonds flavin covalently. For this reason it will be necessary to make use of known procedures for separating these two enzymes, at least in the case of the liver. Although unknown at present, it is assumed that the same would be true for the tumor tissue. Correlation of the enzyme activity and the radioactivity would be determined for the MAO also.